Decline a Phone Interview
If you are asking 'How do I cancel my phone interview?' then I am presuming that you no longer have interest in the position for the job role you applied for and being apart of its recruitment process any longer -- whether that be due to you finding a new job or simply no longer hold an interest for it. But, I also presume that you want to keep a rather professional and good understanding (i.e excuse the expression, not to burn any bridges) between you and the employer at the same time. If so, then you are in the right place. Below, is a brief plan of action, to effectively and politely decline the phone interview, and keep a positive relationship between you and the company (just in case!).
To go about it in a professional way, I would advise phoning up the employer's contact details with which you arranged the phone interview or from the company's website and simply tell them the truth, again whether that be you have been offered a new position that you couldn't refuse or for any other valid reason. They will appreciate that you have taken the time to ring in and saved them the time from calling you, as well as your honesty with them (most simply ignore the phone interview call altogether). Also, be sure to thank them for their time and effort for reviewing your job application and resume as well as being offered a phone interview in the first place.
If you are unable to get in contact with anybody over the phone after several attempts, I would carry out an email (sent to anyone you believe would be relevant) detailing, that firstly you tried contacting them over the phone but ultimately that you are wanting to have your phone interview cancelled, as well as specifying the date that it was arranged. Explain briefly in the email like you would have done over the phone that you have X reason (e.g. on reflection you didn't feel that the job was right for you), and again be sure to be thankful for their time and effort for going over and considering your application/resume as well as being offered a phone interview.
If the employer fails to take note and the phone interviewer does still end up ringing you despite your efforts, greet them professionally but explain at the very beginning of the phone interview the situation. Firstly, that you have attempted to ring in as well as sending an email requesting to cancel the phone interview due to whatever valid reasoning, and again be sure to thank them for their time.
But Before You Do Cancel the Phone Interview
You may want to consider the following -- It may be worth going ahead with the phone interview anyway as although you aren't interested in job you are applying for, you can declare that towards the end of the phone interview something like "the company sounds really great and would be really interested in working for, however the job role of X and its day-to-day activities isn't something that really appeals to me, instead I would be looking to do more X, Y and Z", there then maybe a chance that you impressed so much over the phone interview, that the company offers you a job in a role that you feel is more suited to you. Plus, the fact you are now in a stronger means of negotiation over the compensation package of the job (if you do have a new job lined up) and besides, at the very least it is a bit more phone interview practice. If you have any comments regarding 'How do I cancel my phone interview' or any past experience then I encourage you to please share them below.